In recent years, with miniaturization of electronic devices, a demand for high capacity secondary batteries has increased. Particularly, attention has been attracted to a lithium ion secondary battery having a high energy density and excellent large-current charge-discharge behavior as compared with nickel-cadmium batteries or nickel-hydrogen batteries.
It is known to use graphite as a carbon material for the lithium ion secondary battery. Particularly, when graphite having a large graphitization degree is used as a carbon material for the lithium ion secondary battery, a capacity close to 372 mAh/g that is a theoretical capacity for lithium occlusion of graphite is obtained and furthermore, costs and durability are also excellent, so that it is known that the graphite is preferable as an active material.
However, when graphite is only applied to the lithium ion secondary battery, it is difficult to increase the energy density, obtain the large-current charge-discharge behavior, and exhibit durability characteristics represented by cycle characteristics and applicability of a paste simultaneously.
For example, Patent Document 1 discloses that a lithium ion secondary battery using an aqueous binding agent can be also charged fast by subjecting graphite particles to a mechanochemical treatment and thereby hydrophilizing surfaces of the graphite particles.